The present invention relates to apparatus for controlling the stress in a running string, and more particularly to apparatus used on or in connection with a floating vessel for maintaining the strain in a running string, such as a pipe string, substantially constant while being used in the performance of diverse functions in a sub-aqueous well bore, such as drilling and completion operations therein, despite vertical movement of the vessel while such operations are being performed.
In the normal operation of drilling a well bore on land, or from a drilling platform supported in a fixed position from the ocean floor, the weight on the drilling bit is equal to the total weight of the drilling string less the weight of the drill pipe carried by the drawworks. Usually, the weight imposed on the bit is equal to the weight of the drill collar sections connected to the lower end of the drill pipe. In drilling a sub-aqueous well bore from a floating vessel, the heaving of the vessel under tide, wind and wave conditions introduces problems of maintaining the drilling weight on the bit at the desired value. It is desirable that the drill string be maintained at uniform tension and that variations in tension be minimized in order to carry on normal drilling and well completion operations, prevent undue stressing of the drill string, uneven drill bit pressure, and excessive wear on the drilling equipment, as well as to maintain a fixed in-hole drill string elevation for landing casing, tubing, setting packers, cementing, reaming and other operations requiring close elevation control.
In addition, the vertical movement of the drill string causes the shifting of stresses in the drill collars where the point of neutral stress (change between compression and tension) may cross a drill collar or pipe joint causing undue stressing of such joint and possible failure. Such constantly changing stress at drill collar sections may cause deformation of the drill collars and produce unnecessary cutting of sides of a hole or hole deviation by the resultant dissipation of drilling energy into a lateral direction instead of a vertical direction.
Efforts to cope with the heave problems have produced two principal forms of compensating apparatus. One form is carried by the traveling block and comprises a power cylinder arrangement with pressure ballast to provide force to equal hook load. The traveling compensator system works but adds to the ton miles of work done by the draw works. A second compensator concept comprises a ballasted support for the crown block which moves it vertically relative to the derrick and allows it to maintain a practically uniform distance between crown block and sea bed. The draw works is mounted on and moving with the heaving vessel and complicates the situation. The greatest drawback, however, is the added structural mass high in the derrick. More is required of a vessel for it to remain within stability limits with the extra weight aloft.
The technical burden of the currently available compensator systems is constant whether they are active or passive, in terms of the dead weight present far above the vessel metacentric height.
Also one of the main disadvantages of compensating devices where the drill string is supported hydraulically is the vessel motion dependent behavior of the hydraulic spring that is supporting the load. If the vessel moves the compensating force changes considerably. Motion compensating devices have been proposed for overcoming the aforenoted difficulty. Such types of apparatus, and similar apparatus, are illustrated in U.S. Pat. Nos. 3,714,995, 3,791,628, 5,894,895, 4,423,994 and 4,620,692. In general, the devices illustrated therein rely upon additional controlling devices or complex mechanical solutions to de-couple the motions of the vessel and the resulting compensation force.
With hydraulically supported compensation devices the compensation cylinder acts as a spring with a very low stiffness. Since the “spring” does not have an indefinitely low stiffness a certain variation in the line tension is always necessary to retract or extend the compensation cylinder. This makes the system unsuitable to compensate for loads that are hanging free. This is a disadvantage. The so called “active heave” compensation systems are able to work with loads that are hanging free. Current systems that are used have several disadvantages. The systems are often heavy since they need to support the full load and consume a lot of energy when compensating. Often “active heave compensation” is done using the single drawworks winch, with no redundancy in case this winch breaks down.
It is therefore an object of the invention to provide a compensation system in which the compensation force is not influenced by the motions of the vessel and that is suitable to compensate for the motions of the vessel even when the load is hanging free without excessive power consumption during use and with build in redundancy.
It is furthermore an object of this invention to provide a simple compensation system that does not need to be installed at the top of the derrick.
Also it is an object of this invention to provide a compensation system that is capable of holding a constant tension on the drill bit regardless of the motions of the vessel.
In the devices according to the prior art it is customary for a hoisting cable to be attached to a fixed point at one end. The other end of the hoisting cable is then wound around a winch. If this winch breaks down, it is no longer possible to work with the device. The mentioned winch has also to be of relatively large and costly design to meet with all the required demands. Repeated bending at the same places is a major factor of wear of the cable during normal hoisting and drilling operations and especially when the system is heave compensated. To increase the service life of the cable after a known number of lifting cycles the cable is shifted to move the places of repeated bending. In hoist systems known from prior art this is done by a procedure known as the “slip & cut” procedure. This takes considerable time and is not without personal danger.
It is therefore an object of this invention to provide a hoist system by mechanism of which an increased level of redundancy is provided. It is another object of this invention to provide mechanism with which the time consuming and dangerous “slip & cut” procedure can be avoided altogether. An object of this invention is to provide a hoist system with relatively inexpensive winches decreasing the building and operating cost of the hoist system.
It is therefore advantageous for the hoisting mechanism to be provided with two winches, each end of the hoisting cable being wound onto a separate winch. By winding the two ends onto a separate winch, it is possible to achieve the same cable speed at a relatively low speed of revolution of the winches. By using two winches the cable can be shifted automatically a distance from one winch to the other winch replacing the “slip & cut” procedure. This takes considerably less time and can be performed completely automatic reducing the chance of personal injuries. It can even be performed during compensation operations.
Moreover, by adding the second winch, redundancy is provided in the system. Should one of the winches fail, then the hoist system is not unusable, but it is possible to continue working with a single winch.
It is advantageous for the winches to be driven by a plurality of relatively small motors. Because of the fact that twice as many sides of the winches can be used to attach the motor on these motors can be relatively small. For example, it is possible to equip the winches on both sides with electric motors that engage with a pinion in a toothed wheel of the winch. First, this has the advantage that such electric motors are commercially available. For the use of the hoist system it is therefore not necessary to develop a special, and therefore expensive, hoisting winch. Secondly, the relatively small motors have a low internal inertia, which mechanism, for example, that when the direction of rotation of the winch is reversed less energy and time are lost during the reversal.
In the case of a hoist system according to the prior art of the type mentioned in the preamble, finding the optimum compromise between speed and power is a known problem. The hoisting cable is guided in such a way over the cable blocks in the base structure and on the trolley that several cable parts extend between the base structure and the trolley. In this case the more wire parts are present between the base structure and the trolley; the greater the load that can be lifted with the hoist system if the hoisting winch remains unchanged. However, the more wire parts are present between the base structure and the trolley, the lower the speed at which the trolley can be moved relative to the base structure when the maximal speed of the winch stays the same.
In order to find a good compromise between speed and lifting power, it is generally decided to provide the hoist system with relatively heavy winches. The heavy winches ensure that the requirement of being able to move the trolley up and down rapidly can be met in every case. However, that also mechanism that a substantial part of the lifting power is not being utilized for a substantial part of the time. In other words, the device is actually provided with too heavy—and therefore too expensive—winches to be able to reach sufficient speed occasionally.
It is therefore a further object of the present invention to provide a hoist system of the type mentioned in the preamble. By mechanism of which, on the one hand, a relatively heavy load can be lifted and on the other hand, a relatively light load can be operated at a relatively high speed, while the hoisting mechanism can be of a relatively light and cheap design.
The object is achieved in the present invention by the fact that the hoisting cable is also guided over loose pulleys that can be moved between a first position, in which the loose pulleys are connected to the base structure, and a second position, in which the loose pulleys are connected to the trolley.
The effect of this measure is that the number of wire parts between the base structure and the trolley can be set as desired. When the loose pulleys are attached to the base structure, few wire parts will extend between the base structure and the trolley, and a relatively low weight can be lifted with a relatively high speed. When the loose pulleys are attached to the trolley, a relatively large number of wire parts will extend between the base structure and the trolley, and the trolley can be moved at a relatively low speed relative to the base structure with a relatively large load. Since the hoisting cable is guided over the pulleys and the pulleys can be attached as desired to the base structure or to the trolley, the hoisting cable does not have to be reeved again. That mechanism that the desired number of wire parts can be set in a relatively short time.
It is possible according to the invention for the loose pulleys to be attached symmetrically relative to the center of the base structure.
This ensures that the forces exerted upon the cables are also transmitted symmetrically to a base structure, which mechanism that no additional bending loads are exerted upon the base structure limiting the necessary weight of the base structure.
It is possible according to the invention for the loose pulleys to be accommodated in a housing, which at least on the bottom side is provided with locking elements for fixing the pulleys on the trolley. The loose pulleys are pulled automatically into their first position, in contact with the base structure, by tension in the hoisting cable. It is therefore sufficient to provide the bottom side of the pulleys with locking elements.